Place a pry bar, nail puller, tube of roof cement, caulk gun, roofing nails and a hammer in a bucket.
Place a ladder against your house near the water heater vent installed on the roof.
Carry your bucket of tools onto the roof with you up the ladder.
Pull back the shingles surrounding the water heater vent using your hands and a pry bar. Pull the shingles up enough to expose the nails fastening them in place. Pull the nails out with a nail puller to free the shingles.
Insert the tube of roof cement into the caulk gun and spread roof cement around the base of the vent, filling any visible crevices or holes.
Put the shingles back into place around the vent and nail them to the roof using roofing nails and a hammer. Monitor your ceiling when it rains to ensure that no more water is leaking from around the water heater vent.